In an electric power steering apparatus according to the prior art shown in FIG. 8, a lower column 102 is fitted in a vehicle-front-sided portion of an upper column 101 of a steering column, and an upper shaft 103 and a lower shaft (input shaft) 104 of a steering shaft that are spline-fitted to each other are rotatably supported in these columns 101 and 102.
An output shaft 105 is connected via a stopper portion 106 to a vehicle-front-sided portion of the lower shaft (input shaft) 104. A steering gear (unillustrated) is connected via a universal joint (unillustrated), etc. to a vehicle-front-sided portion of this output shaft 105.
The output shaft 105 is supported in housing 109 and in housing 110 through a pair of bearings 107, 108. A proximal end of a torsion bar 111 is fixedly press-fitted in a vehicle-front-sided portion of the lower shaft (input shaft) 104. This torsion bar 111 extends through an interior of the hollowed output shaft 105, and its front end is fixed to an end portion of the output shaft 105 by a fixing pin 112.
Torque detection grooves 113 are formed in the vehicle-rear-sided portion of the output shaft 105. A sleeve 114 of a torque sensor is disposed outwards in a radial direction of these grooves 113. A vehicle rear-sided end portion of this sleeve 114 is fixed to the vehicle-front-sided end portion of the lower shaft (input shaft) 104 by caulking or the like. Coils 115, a board, etc. are provided outwards in a radial direction of the sleeve 114.
The output shaft 105 is fitted with a worm wheel 118 meshing with a worm 117 defined as a drive shaft of an electric motor 116.
Accordingly, a steering force generated when a driver steers the steering wheel (unillustrated) is transferred to unillustrated steered travelling wheels via the input shaft 104, the torsion bar 111, the output shaft 105 and a rack-and-pinion steering gear device. Further, a rotational force of the electric motor 116 is transferred to the output shaft 105 via the worm 117 and the worm wheel 118. An adequate amount of steering auxiliary torque can be applied to the output shaft 105 by properly controlling the rotational force and a rotational direction of the electric motor 116.
As described above, in the electric power steering apparatus shown in FIG. 8, the sleeve 114 takes a structure that the vehicle rear-sided portion of the sleeve 114 is fixed to the vehicle front-sided end portion of the lower shaft (input shaft) 104, and hence axis-directional lengths of the housing 109 and of the housing 110 for the electric power steering are comparatively taken long.
As a result, the axis-directional lengths of the housing 109 and of the housing 110 specify a collapse/stroke quantity of the upper column 101 when a secondary collision of the vehicle happens. Therefore, the collapse/stroke quantity can not be increased over the stroke quantity shown in FIG. 8 in spite of a demand for increasing this stroke quantity.
Further, in the electric power steering apparatus, the steering auxiliary torque of the electric motor 116 is applied to the output shaft 105 via the worm wheel 118, and consequently larger torsional torque than on the input shaft 104 disposed on the rear side of the vehicle occurs on this output shaft 105 when in a steady state.
Moreover, if a reverse input, e.g., an impact force generated as when a tire collides with a curbstone acts on the output shaft 105 from the steering gear side, the worm wheel 118 has motor inertia that increases with a square of a gear ratio, and hence the extremely large impact torsional torque occurs on the output shaft 105.
The electric power steering apparatus according to the prior art shown in FIG. 8 has the hollowed structure that the torsion bar 111 extends through within the output shaft 105, and the torsion bar 111 is fixedly press-fitted in the end portion of the output shaft 105. Hence, if a small-diameter portion 105a of the output shaft 105 connected to a universal joint of the steering gear is twisted by the aforementioned torsional torque to an extremely slight degree in excess of an elasticity limit, its torsion (rotation) is detected by a torque sensor and appears to be an output deviation of the torque sensor, wherein this output deviation might induce self-steering. It is therefore required that strength of the small-diameter portion 105a of the output shaft 105 be improved by effecting a thermal treatment upon this small-diameter portion 105a. 